Unified view of low- and high-frequency regimes of atomic ionization in intense laser fields

Abstract

On the basis of the Floquet formalism, a two-dimensional model atom in circularly polarized intense laser fields is analyzed. By solving the close-coupling equations, the pole positions of the scattering matrix are calculated on the complex quasienergy Riemann surface, and the pole trajectories with respect to the variation in the laser intensity are obtained for different laser frequency regimes. The behaviors of the pole trajectories indicate that the tunneling ionization is typically observed in the low-frequency regime, while the stabilization of the atom occurs in the high-frequency regime. The mechanisms of the atomic ionization in these two regimes are discussed from a unified point of view. The transition between the different frequency regimes can be explained by the change in the avoided crossings among the adiabatic potential curves for the radial motion of the electron.